because of the mismatch between the millennial persistence time of carbon dioxide and the sub-decadal persistence of stratospheric particles, albedo modification can never safely play more than a very minor role in the portfolio of solutions. There is simply no substitute for decarbonization.

The problem is that CO2 accumulates in the atmosphere, and this enhancement in atmospheric CO2 will persist for thousands of years. Hence, while we continue to emit CO2 it will continue to accumulate in the atmosphere, the climate will continue to change, and these changes will remain for many, many generations.

Solar radiation management works by injecting aerosol particles into the stratosphere that then reflect some of the incoming sunlight, hence the term albedo modification. The problem is that these aerosols precipitate within a few years. Hence, to maintain this would require continually repleneshing these particles. To counter-act the effects of climate change, we’d then need to continue doing this for thousands of years: a co-ordinated global effort that would need to last for longer than we’ve been able to maintain any advanced civilisation so far.

If we failed to maintain SRM, the particles would precipitate and we’d undergo what is known as a termination shock; the climate would catastrophically warm over a period of a few years. There are also potential geo-politcal issues. What if a powerful country decided that they preferred it slightly warmer. Maybe another would prefer to adjust it slightly so as to change the rainfall patterns in their region, but that such changes would then negatively impact another. The point is that something like solar radiation management isn’t a really viable plan B; we really need to focus on decarbonising the global economy.

However, a number on Twitter disagree and argued that even if it is something that we may never want to use, we should at least understand how it might work. If we do fail to sufficiently decarbonise the global economy then we may indeed want to consider something like solar radiation management. Additionally, the problems highlighted above may not be too big an issue if it’s only used to counteract a relatively small amout of global warming.

This, however, seemed to create another level of criticism. Essentially, solar radiation management will have a global impact and so studying it, and potentially implementing it, really needs to be inclusive; it can’t just be the wealthy developed world who decide if it’s something worth considering. Similarly, if we’re going to justify its use on the basis that economic growth in the developing world is likely to require increasing use of fossil fuels, we really should check if this is the path that such countries would like to follow. If we really do care about the developing world, maybe we should instead consider making the kind of sacrifices in the developed world that would lead to emission reductions, rather than studying a possible plan B.

I had intended that this might be some kind of open thread, but have written a bit too much. My own view is that Ray is essentially correct; there really is no substitute for decarbonisation. I don’t, however, have a major problem with studying a possibly geo-engineering solution, but I do think we have to be very careful that it doesn’t end up being used as an excuse to delay emission reductions and we would really need to ensure that the engagement with this is inclusive and not dominated by a minority in the developed world. Would, however, be interested to hear what other people think.

50 Responses to Solar Radiation Management

It’s worth noting that the most recent meeting of the Geoengineering Model Intercomparison Project (GeoMIP) had a special focus on modellers of potential SRM impacts from developing nations.

I would also say, having attended the meeting, that not one person researching this views it as a substitute for decarbonisation. But if we fail to decarbonise rapidly, or climate sensitivity is higher than we expect, SRM may be the only technique that can prevent catastrophic temperature rises in time for decarbonisation to take effect.

My concern about SRM, and it seems all the more every month about mitigating climate disruption, is that the public refuse to educate themselves enough about what’s going on to make any intelligent choice, let alone a risk-balanced one. I worry, in the case of SRM, that it might be the default because of its comparatively low cost, even though it has dangers, even though it might not work as well as people think, even though there’s the risk of termination shock, even though it won’t do anything for ocean acidification, even though it may make solar PV operate less efficiently, even though it has unknown health consequences.

I am reading Professor Keith’s book, and note that the late Prof Wally Broecker (see his recorded lectures) felt SRM might be necessary. I remain unconvinced, at least as yet. It’s unfortunate we collectively are in this position.

My final worry about SRM is for future scientists and engineers … Trotted out and misunderstood as a supposed solution, if something were to go badly wrong, they might be held accountable. How do you shield them from liability?

I would also say, having attended the meeting, that not one person researching this views it as a substitute for decarbonisation.

Yes, I realise this. Should probably have made this clearer.

But if we fail to decarbonise rapidly, or climate sensitivity is higher than we expect, SRM may be the only technique that can prevent catastrophic temperature rises in time for decarbonisation to take effect.

One thought I did have (and would be interested in your views) is that if we got to the point were some kind of interventation became seriously considered, wouldn’t we simply go ahead and do it? Don’t we essentially know enough know about SRM to broadly know what would happen if it were used? In other words, if it became clear that climate change were becoming catastrophic wouldn’t we simply use SRM? Why do we need to do more research? Is there some chance that the research might indicate that under no circumstance should we implement SRM?

As Clive noted, geoengineering is a daft idea and SRM does nothing to address ocean acidification as EcoQ noted. But I think it is likely that we will see ad hoc attempts at geoengineering like SRM nonetheless. We have done so much inadvertent geoengineering at this point with GHG emissions, that it only makes sense that some of our species’ nation states will decide it makes sense to try deliberate geoengineering. Put your trays in the full upright position, there is turbulence ahead and the ride is likely to get bumpy.

Your Twitter pal could be right about one thing, although in an irrelevant way. Summer high Arctic temperatures probably have not warmed very much and that’s mostly because excess energy is used up by phase transitions from ice to water having passed the melt threshold, which puts a cap on temperature even as the amount of ice melting increases.

> Summer high Arctic temperatures probably have not warmed very much and that’s mostly because excess energy is used up by phase transitions from ice to water having passed the melt threshold, which puts a cap on temperature even as the amount of ice melting increases.

One thing at a time, PaulS. He has yet to read the Wiki page on Arctic amplification or acknowledge that his own source (DMI) observed a 8 DEGREE CELCIUS increase above normal in recent years:

TRUE OR FALSE – Mike asks leading questions and does science by stock photo instead on reflecting on where goes the heat that has been observed? pic.twitter.com/FTna5kHs3B

If we failed to maintain SRM, the particles would precipitate and we’d undergo what is known as a termination shock; the climate would catastrophically warm over a period of a few years.

It happened below ATTP. It even has a name. The Great Dying. There’s increasing evidence that there was a brief glaciation at the onset of the end-Permian mass extinction, and multiple cooling and warming episodes during the early Triassic, which was generally warmer than the late Permian and marked both by further extinction events and very slow (million-year-timescale) biotic recovery. All coinciding with eruption of the Siberian Traps. The end Triassic, another extinction event which paved the way for the spread of the dinosaurs, was also large-igneous province related and had a delayed recovery due to extreme ecosystem instability. And the end-Ordovician, probably the first to be identified as an ice-fire double-whammy, follows the same pattern.

Seems like, for life on Earth, the only thing worse than an ice age or global warming, is an ice age and global warming, in quick succession.

I agree with you, as a mech.eng. ought to know about thermal capacity of water and energy involved in phase transitions.

On the other hand — and thank goodness — the thermal capacity of water is a truly amazing thing, and it’s easy for people to miss its implications. I’d say if 90%-95% weren’t going into the oceans, we’d have a real problem. On the other hand, they are going to hang onto it a long time.

There are a few of these things in physical oceanography. This is why, in part, I include in an introductory class when I teach the definition of a Sverdrup.

Just a thought, but although this is probably correct, there's – IMO – an asymmetry. Our imperfect knowledge of the GHG effects are, in a sense, informing what we might want to avoid. We may want better knowledge if we were intending some kind of active climate management. https://t.co/4OXZlvAcvd

Given the world’s inability to pull in one direction on CO2 emisssions, why should we expect that SRM will be any more coordinated? Also, there will be losers even if the world gets its act together. As usual, there will be a political push to put the bad effects on someone else. I don’t see it happening.

is it possible that some of the current warming period is in fact a ‘mini’ termination shock in response to our anti pollution efforts?

Almost certainly some warming if global effective anthropogenic aerosol forcing has weakened. It’s widely expected that future clean air initiatives, even in no-climate-policy scenarios, will cause a decline in particulates which will contribute to future warming, though it would be difficult to describe as a shock since the change would be gradual. To date global particulate trends are fairly flat overall, with declines in developed countries being countered by increases in developing countries, so there’s uncertainty over whether or not overall aerosol forcing has weakened.

A possible analogue for a regional shock could be 1930s USA and North Atlantic. The crash and Great Depression is estimated to have caused a near-halving of SO2 emissions from the US over just 2-3 years from 1929 to 1932.

FYI
CNN will host 7 hours of interviews with most major Democrat primary candidates solely on the issue of climate change. It’s tonight 5 p.m. to midnight-ish eastern US time.
Candidate plans run the gamut of ideas from Bernie Sanders’ “economic transition” similar to the Green New Deal to Andrew Yang’s proposal to subsidize build-out of thorium reactors and everything in-between.
It appears from CNN that it will be carried on CNN International.

Tom, I thought everyone knew that the unmitigated aerosol pollution before we started cleaning smokestacks and reducing smoky coal fires has masked post-WWII warming. Of course the CO2 didn’t go away, and has contributed to the warming since the aerosols rained out.

And in case anyone was wondering, no it doesn’t mean ECS has been over-estimated and is lower than the IPCC range. The effect has been known about for decades and is already factored in.

It seems to me that SRM would reduce incoming radiation where the sun shines (tropical regions) more than where it doesn’t (polar regions). GHGs, on the other hand, tend to be more even handed, though they’ll proportionally reduce the outgoing radiation in polar regions more (because the air’s drier there [¹]).

Therefore, even if the net effect of GHGs and SRM is the same temperature averaged over the planet the tropics would be cooler, the polar regions warmer and the flow of heat from the tropics towards the poles would be decreased. Given that that flow is the major driver of weather I wonder how much effect this reduction would have on weather patterns. Is there any indication of this from volcanic activity?

> , there’s – IMO – an asymmetry. Our imperfect knowledge of the GHG effects are, in a sense, informing what we might want to avoid.

What is the asymmetry? Assuming that we had very high confidence of something > 3c…

… and that there we have a lot of uncertainty as to the effects of something like SRM…

Presumably the negative ramifications from SRM would be reversible on a short time scale whether by design or because of the difficulty of maintaining the intervention. The impact of >3c not so much. What is the recipe for decision-making amid those uncertainties?

is it possible that some of the current warming period is in fact a ‘mini’ termination shock in response to our anti pollution efforts?

There has been some suggestions that some of the post WWII cooling was because of enhanced aerosols. So, some of the warming after that could have been due to them precipitating. However, the aerosol forcing is still negative, so aerosols are still providing a cooling influence. If they were to all precipitate, then we’d expect a few tenths of a degree of warming over a decade or so. However, the cooling influence of aerosols is about the same as the warming influence of short-lived GHGs. Hence, of the aerosols were to all precipitate, and the short-lived GHGs were to all decay, there would be little net effect. This figure from the IPCC SR1.5 report illustrates this (the precipitation timescale of the aerosols is shorter than the decay timescale of the short-lived GHGs).

What I mean is that the uncertainty in the projections of climate change may lead us to conclude that we really should aim to minimise our emissions. We don’t need a high level of certainty to make that decision. On the other hand, if we’re going to actively do something (such as inject aerosol particles into the stratosphere) we may want a reasonable level of certainty before doing so.

I believe SRM is a bad idea and I believe our species will employ it on an ad hoc basis as a solution to global warming. One of the problems with SRM is that it treats a symptom (warming) of the problem (ghg accumulation). It seems silly to me to treat the symptom when we need to be treating the problem, but I recognize that it might buy us some time to improve action on emissions related to building heat. Absent an underlying commitment and significant action to reduce ghg emissions, SRM seems like a very bad idea.

There is also the issue that SRM does nothing to address ocean acidification (thanks for bringing up quickly EQ). Earth is primarily an ocean world. I think if the oceans are not healthy, the planet is not healthy. Canfield oceans, anyone?

I continue to focus on one primary indicator (CO2 in the atmosphere) and I see no significant progress on changing the trajectory of this difficult problem. I realize this single issue focus is a bit reductionist, but in addressing a problem that has great complexity and potential for distraction and misdirection, CO2 accumulation in the atmosphere is ground zero for me and I return to it over and over.

I encourage folks who are looking for good news to be tolerant and respectful toward those of us who have lost energy for activism and are considering the possibility and impact of civilization collapse.

I pledge to be tolerant and respectful toward those of you who retain hope and energy for activism and think we are making significant progress on the problems of CO2e emissions. If you think/believe/have faith that we can avoid civilization collapse, I am happy for you and I hope you are correct. I won’t question your faith or belief systems. Those are yours and I respect your right to them. I hope for a tolerant and respectful discussion of the science, facts and evidence regarding the problem of CO2e emissions.

No doubt, it’s ALL about CO2 emissions, that they — and probably N2O — need to be zeroed, and that, actually, the responsibility for paying for this, if fairness is any kind of a consideration, belows to those and in proportion to the ownership of cumulative greenhouse gas emissions.

Alas, direct air capture of CO2 is incredibly expensive. A bunch of people are enthusiastic about planting lots of forests, but these have limits (see section 9.1) and — White Supremacy to the Fore — whose country are they volunteering to host these? (By rights they should be planted right in the middle of the good ole USA.) The thing about SRM is, compared to other remedies, it’s really cheap. It’s even cheaper than converting entirely to zero Carbon energy.

I think people are, collectively speaking, really cheap. Even if it’s stupid.

@Majura Wombat, about “the effect of SRM on photosynthesis and hence food production and natural CO2 sinks.”

There’s experimental evidence that making light more diffuse actually increases photosynthesis in plants, just by increasing the light reaching leaves that might be in shade were the light more direct. A bit more efficiency seems to be from lowering temperatures slightly, too. Here’s a paper about it from Annals of Botany in 2014. https://doi.org/10.1093/aob/mcu071 That was a controlled greenhouse experiment with tomato plants, checking only the difference between direct and diffuse sunlight.

This abstract for a 2018 paper in Nature estimates from natural experiments (big stratospheric volcanic eruptions) that the benefit of SRM vs the harm would be a wash by mid-century conditions – though the abstract mentions C3 and C4 grain crops, not fruit or vegetable crops. And contrary to the “Annals” paper, it says diffusion is the cause of decreased output. Since grains are our biggest food source, the effect might be good for trees, good for non-grass crops, but still hurt our overall food production.https://www.nature.com/articles/s41586-018-0417-3/.

The other part of SRM that scares the bejeebers out of me is what if someone starts a program (hey, it’s cheap!) and then a volcano adds to the dimming? Or we find out that several groups did at once, or that a global crop year or two is lost as we try fine-tuning. “What could go wrong?” is my least favorite sentence when talking about things like this.

If climate sensitivity is ‘Nik Lewis’ low, then SRM becomes twice as hard, and costly as it would be if climate sensitivity is high.
Twice as much forcing change to cool things if TCS is 1.4 instead of 2.8

Then do not use a climate intervention as a substitute for decarbonization, but as a substitute for suffering the consequences of climate change.

Once we have decarbonized the economy and did it according to the current political plans, which mean a warming of 3°C to 3.5°C, would people here at that moment still object to a climate intervention?

Victor,
I suspect that that will depend very much on how bad it turns out to be at 3C or 3.5C and how much intervention is planned. I would guess that trying to counteract all of the 3C – 3.5C of warming might be a very bad idea, but doing something to reverse some fraction of this might be worth trying (I know it’s not strictly reversing, but I can’t think of a better word).

I have seen a study which had a scenario to use solar radiation management to half the warming. This would mean nearly no warming in the tropics (where SRM does most and the greenhouse effect does least). Could be preferable over heatwaves were mammals can no longer survive outside.

VV said: “I have seen a study which had a scenario to use solar radiation management to half the warming.” That was probably this one: https://www.nature.com/articles/s41558-019-0398-8.epdf
It’s cheap, we will probably do it. What could go wrong? I think we may find out.

In the comment I made above with the link to Wally Broecker’s talk at BU, he mentions that when he first looked at sulfate-based SRM and tried to get a price on it, he inquired and the expert on sulfur said at the time (1980s) Broecker was talking about taking the entire world’s production each year. I don’t know what David Keith of Harvard says about this these days, but there could be other limits, too.

There are also proposals to have drone ships cruise the oceans and spray water in fine droplets up so as to whiten the ocean surface. Indeed, Sir David King has a proposal to use that kind of thing to try to refreeze the Arctic. King is big on worrying about deleterious effects from massive Arctic amplification.

@ecoquant: that’s an interesting video, although my attention span is not quite up to 30 mins.

At least for the sulfur-based solutions, the forcing goes up quite slowly with concentration, so if you only want 0.5-1W/m^2 maybe it isn’t so bad. But indeed if you need much more you need to start putting all the world’s sulfur into the upper atmosphere that is expensive and dirty.

So I guess this is obvious to all the experts, but SRM is playing around at the edges: the 6W/m^2 or so of reduction we need probably have to come almost entirely from reducing CO2 emissions.

That is, the problem is not just that we have to maintain SRM for long timescales, but that it is hard to have a big enough effect with SRM compared to CO2.